The present invention relates to systems for controlling refrigerant cooling of a vehicle compartment such as the passenger compartment of an automobile. The system relates particularly to air conditioning systems for automobiles having a compressor/pump which circulates refrigerant between an exothermic heat exchanger, or condenser as it is commonly called, and an endothermic heat exchanger or evaporator located to have air blown thereover for cooling the passenger compartment. In systems of this type, flow control of the compressed and liquified refrigerant from the condenser is maintained by an expansion device such as a throttling capillary tube or an expansion control valve located to receive refrigerant flow from the condenser at its inlet and to discharge vaporized refrigerant at its outlet at a substantially lower pressure for circulation through the evaporator. Systems of this type are well known in the art and generally employ pressure sensing devices disposed in the flow path of the refrigerant to the evaporator or in the flow path of the refrigerant discharging from the evaporator to effect operation of the control valve or cut off of the compressor in response to excessive pressure in the refrigerant line.
One such commonly employed type of refrigerant expansion control valve employs a diaphragm exposed to refrigerant pressure leaving the valve. A bulb senses the temperature at the evaporator outlet and in response thereto, pressurizes a fluid filled chamber acting on the opposite side of the diaphragm. Movement of the diaphragm in response to the pressure differential thereacross, is employed to move a refrigerant flow control valve member. Such pressure responsive refrigerant expansion control valves thus function to control the flow of liquid refrigerant to the evaporator in response to demand, or rate of vaporization in the evaporator. The aforementioned pressure responsive valves have found widespread usage in automotive air conditioning systems because of their simplicity, low cost and reliability.
However such pressure responsive valves are limited in their control capability by virtue of their responsiveness to only the refrigerant superheat at the evaporator outlet. Thus, such known systems for controlling refrigerant flow in air conditioning systems, require a pressure sensing means in the evaporator flow circuit capable of sensing the differential pressure. Recently, it has been desired to provide an all electrically control}ed air conditioning system for a vehicle, and particularly a system which could integrate the control of the electric compressor clutching mechanism in a common microcomputer employed for engine operating control.
Accordingly, it has been desired to provide a way or means of providing all electrical control of an automotive air conditioning system and yet retaining the advantages of the refrigerant expansion means known in the art. One such known expansion means comprises a pressure responsive diaphragm operated thermal expansion valve such as that shown in and described in U.S. Pat. Nos. 3,667,247 and 3,810,366 owned by the Assignee of the present invention. Alternatively, a simple capillary tube may be employed as an expansion means as is well known in the art.